I first signed up for NaNoWriMo in the year 2004, and started a novel called Sam and Charlie Go On A Roadtrip, which was about two girls, Samantha (Sam) and Charlotte (Charlie) who took a roadtrip down to Florida.
I stalled out at 8,881 words.
My usual line is that I got my characters as far as Florida and got stuck. I've been told that Florida has a reputation for that sort of thing, but there were other factors involved.
1. Not actually having a plot in mind. The novel was written as alternating diary entries and I figured that just rambling about travel from place to place would be an easy way to rack up words. Wrong. That's not even what we read novels for, anyway--we pick up books and keep reading them because we want to know what happens next. If there's no tension, no conflict and no potential gain or loss, there's nothing to drive things forward.
I almost made the same mistake in 2005, starting on a fantasy story that was supposed to be somebody dictating her memoirs to the court historian, but I realized that I had very little idea where it was going, so I tore it up and started a new story about ten days into the month. It was one of the most basic plots in the world--the Cinderella story, effectively--and I wound up crossing the finish line for the first time.
2. Setting it somewhere that needed too much research. For reasons I'll not get into, I thought it would be a great idea to set Sam and Charlie in the year 1990. This was, in fact, a really bad idea because then I had to make sure I wasn't inserting any anachronisms like having them listen to albums that hadn't been released yet. I found myself constantly second-guessing things as I went and was hesitant to go into certain details because then I'd have to fire up the internets and make sure they were accurate.
Again, 2005 almost fell to the same mistake, because I was setting it in a pre-gunpowder setting and was still determining the world it was set in. The story I started after I tore that one up was set on a space station in the far future, so I could pull any damn thing out that I wanted and potentially get away with it.
3. Trying too hard to be cool. By hampering myself to keep things cynical and realistic, I cut myself off from a great deal of passion and excitement.
In 2005, the novel I did finish was a mushy, shameless love story and I held nothing back. (Indeed, I enjoyed hanging out with those two characters so much that they showed up again in 2008 and 2009 and I told the stories of their happily-ever-after.)
While it would have been nice to have a unbroken record of complete success from 2004 on, I am actually glad for my first spectacular failure because it taught me exactly what didn't work, so I've been able to complete NaNoWriMo successfully for every single year ever since.